Image converter tube



Nov. 13, 1951 c. s. SZEGHO 2,575,033

IMAGE CONVERTER TUBE Filed May 1, 1948 x 6 5 Ti 4 3am A 2 x f "33T'lil'lTT'l'l'IIfil'P'T INVENTOR.

CON STANTIN S SZEGHO 4 7' TORNFY.

ment time.

Patented Nov. 13, 1951 lConstantin .S. 'Szegho, Chioago,.-Ill., assignorto 'TheTRauland Corporation, Chicago, 111., .a zcorjporation pf"Illinois Application 1, 1948, Serial No. 24,631

2 'Claims.

This invention'relates to :new and useful improvements inimage convertertubes and more particularly to screens for such tubes.

One well known use of image converter tubes is to reproduce a picture ofa poorly illuminated object on a viewing screen. This is accomplishedby1=focusing whatever light :comes from -the (object onto a photocathodewhich weleases saifiood of electrons varying in density accordance withthe light distribution from the object. Accelerating and focusing meansare provided within the tube to direct the electrons towardsafluorescent screen where a visible image is produced by the impingingelectrons. Usually the density and velocity of the electron flood arelow, despite the use of sensitive photocathodes and strong acceleratingmeans, and the image produced on the fluorescent screen is notsurficiently bright.

Various suggestions have been made for overcoming this shortcoming ofimage converter tubes. According to one, the electrons released from thecathode are multiplied by secondary emission to increase the brightnessof the image as, for example, described in British Patent No.

460,579 to Szegho and Lance.

One of the disadvantages of the above is that the tube is unnecessarilycomplicated as a result of the need for means for producing secondaryelectrons.

According to the invention these and other disadvantages of imageconverter tubes have been overcome and the tube which is the subject ofthis invention is as simple as any image converter tube, yet producesbright clear images of even the most poorly illuminated objects.

This is accomplished by replacing the conventional fluorescent screenwith a screen which is cumulatively responsive to electron impingement.

The length of time a fluorescent screen is exposed to electronimpingement, except for the initial short excitation period, does notvary its luminous output. However, in the case of 2. cu-

mulatively responsive screen it is possible to improve the brightness ofan image produced thereon simply by lengthening the exposure time.

' Such a screen may be formed out of the socalled ionic crystals whichwhen struck by electrons develop a deposit of opaque material. The

" density of this deposit on any given area of the screen variesdirectly with th density and velocity of the electrons and grows withimpinge- The opacity of the screen will, therefore, be enhanced thelonger it is bombarded. The electrons insteadof prodncingdig'htnn thescreen produce .darhness, :tenebrescence. The ,most satisfactory screenshave been .made of potassium chloride vandare .frequentlyreferred to asscotophorscreens.

The decay time of such a screen may run from hours to days andtherefore, it .is possible .to make a semi-permanent recording .of .anobject. However,-.if this is not desirable, according to another featureof the invention it is possible to erase an image produced on the screenby applying, heat or an electric field to the screen. In this way assoon as the usefulness of one image has ceased, the screen may be wipedclean of it and made ready for another image.

According to a feature of the invention, an image may be produced in thedarkness without employin visible light. In this case a specialphotocathode is used which is sensitive to infrared light used toilluminate the object. Such a photocathode is the so-called caesiatedsilver surface or other special emitters of great sensitivity. The imagemay be projected onto a viewing surface or it can be seen directly onthe screen of the tube as a black-White pattern. If there isinsuflicient ambient light to illuminate the screen, light from anauxiliary source may be used for this purpose.

The invention will now be described in greater detail, reference beinghad to the accompanying drawing which shows, by way of example. oneembodiment of the invention.

A semi-transparent photocathode l is mounted on end wall 2 of imageconverter tube 3. Light from an object 4 may be focused by lens 5through end wall 2 upon photocathode End wall 6 of tube 3 carries ascreen I which is cumulatively responsive to electron impingement. Asthe exposure time of screen I to the electrons emitted from photocathodeI is increased, the brightness of the image on the screen improves. Thescreen may be formed of ionic crystals such as potassium chloride. Analuminum film 8 may cover the screen and also part of the side walls oftube 3. Film 8 receives positive potential from source 9 and functionsas an accelerating electrode for the photoelectrons and as a collectinganode for slow-moving electrons.

Two pairs of electrodes I0 and H, to which positive potentials areapplied from sources l2, l3, are situated between photocathode l andscreen 1, and accelerate and focus the photoelectrons emitted fromcathode I; instead of electrostatic means electromagnetic coils may beused for focusing.

In certain cases the decay time of the screen may be too long andtherefore, it is advisable to use heating means such as a source ofcurrent H! which is connected by suitable leads to end wall 6, to erasethe image on the screen. Wall 6 may be formed of conductive glass orhave a conductivelayer supporting the screen to which is connectedsource I4. However, other ways of erasing screen 1 may be employed.

A poorly illuminated object 4 may be illuminated by infra-red light andprojected by lens 5 onto a semi-transparent photocathode I. Theelectrons emitted from photocathode I are distributed according to thedistribution of the light values of object 4. An electric field which 7is set up on rings 10 and II and on aluminum film 8, accelerates theelectrons so'that they travel through tube 3 towards end wall 6 wherethey produce an image'of gradually increasing clarity. Naturally therewill come a point in the exposure time of the screen to the electronswhen increasing definition and clearness give way to obscurity becausethe screen becomes completely covered by the opaque deposits. Therefore,after the image is sufficiently clear on the screen the electronimpingement should be discontinued, as by masking or turning thephotocathode away from the object.

Since the image produced on screen 1 is a dark trace and needsillumination from an external source, such a source of light, generallyindicated at l5, may be provided and positioned to shine on end wall 6and illuminatescreen 1.

What is claimed is:

1. In an image converter tube for producing still images of objects, aphotocathode exposed to light rays emanating from the object, a screencomprising ionic crystals aligned with said photocathode andcumulatively responsive to the impingement of electrons therefrom, meansfor accelerating and. focusing the electrons onto the' REFERENCES CITED'The following references are of record file of this patent:

UNITED STATES PATENTS Number Name Date 2,074,226 Kunz et a1. Mar. 16,1937 2,189,322 Flory Feb. 6, 1940 2,330,171 Rothenthal Sept. 21,1943

